During drilling operations there is a need to build mud cakes as impervious as possible on the wall of the borehole to prevent damage by fluid loss. To establish the relationship between the filtration properties of mud filter cakes and their structure, drilling fluids containing montmorillonite clay, flexible (Drispac) or rigid (xanthan) anionic polymers, electrolytes and barite were investigated. Standard API filtration and polymer adsorption measurements were performed. The cake structure was vizualized by cryoscanning electron microscopy. Our study establishes the mechanism of filtration reduction properties of drilling filter cakes. Filtration measurements and cryomicroscopy show that a well dispersed suspension forms a well connected, homogeneous network that prevents fluid loss. Addition of salts to the drilling fluid promotes the aggregation of the clay particles and introduces heterogeneities so that the fluid loss is increased. Addition of anionic polymers causes a better dispersion of the clay particles in the suspension which lowers the filtration rate. In the presence of both salts and polymers, there is a competition effect between the salt-induced aggregation of the clay particles and the dispersion due to polymers, largely governed by the concentration and valence of the electrolytes and the flexibility of the polymers. During the filtration process more filtration reduction properties are obtained by non adsorbed polymers which clog the pores of the cake network and further reduce the filtration rate. The different behavior of the flexible (Drispac) and the rigid polymer (xanthan), can be used to optimize their applications in drilling fluids depending on field conditions (temperature, salt conditions).